The present invention relates to a signal processing method for Doppler Effect radar speed sensor.
Current vehicle radar speed sensors, such as those used on agricultural tractors, are susceptible to vibration and thus indicate vehicle movement even when the vehicle is standing still. Commercially available vehicle mounted Doppler radar speed sensing units detect motion and generate a speed signal. Such units can generate an inaccurate speed signal when subjected to vibration of the vehicle or when parked in view of waving grass, paper etc. Software filters have been used to attempt to determine if the detected motion is due to actual vehicle speed or to some artifact. Such filters act to slow down the response of sensor. Another method used to overcome this problem is to have two radar units (two antennas, two detectors) mounted such that they view the ground in different directions (the so-called xe2x80x9cJanusxe2x80x9d configuration). A signal processing system receives the signals from both radar units and determine whether or not the vehicle is actually moving.
Accordingly, an object of this invention is to provide a radar speed sensing system which provides accurate speed information in spite of vehicle vibrations.
A further object of the invention is to provide such a radar speed sensing system which provides both speed and direction information.
A further object of the invention is to provide such a radar speed sensing system which does not require a slow performing software filter.
A further object of the invention is to provide such a radar speed sensing system which does not require two antennas and two detectors.
These and other objects are achieved by the present invention, wherein a radar speed sensing system includes a microwave transceiver which has two detectors arranged so that they generate a pair of Doppler shifted signals which are 90 degrees out of phase with each other. A digital signal processor (DSP) processes the received Doppler signals. The DSP executes a complex fast Fourier transform (FFT) routine which allows both the direction and speed of the sensor to be determined simultaneously. Direction is sensed by determining if the phase on the first Doppler signal lags 90 degrees behind or leads 90 degrees ahead, of the other Doppler signal. With the DSP running an FFT routine, speed is determined in the frequency domain, instead of the time domain. Determining speed in the frequency domain results in a system which is better able to distinguish noise from an actual Doppler signal. The complex FFT can identify frequencies caused by vehicle vibration, which can then be ignored or discounted. By determining vibration frequencies, this system is able to avoid indicating vehicle movement when the vehicle is standing still.